Earth and Planetary Science Letters 432 (2015) 354–362

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Earth and Planetary Science Letters

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Teleseismic shear-wave splitting in SE Tibet:Insight into complex crust and upper-mantle deformation

Zhouchuan Huang a,b,∗, Liangshu Wang a,b, Mingjie Xu a,b, Zhifeng Ding c, Yan Wu c, Pan Wang a,b, Ning Mi a,b, Dayong Yu a,b, Hua Li a,b

a State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210046, Chinab Institute of Geophysics and Geodynamics, Nanjing University, Nanjing 210046, Chinac Institute of Geophysics, China Earthquake Administration, Beijing 100081, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 27 May 2015Received in revised form 15 October 2015Accepted 17 October 2015Available online 3 November 2015Editor: A. Yin

Keywords:Tibet and YunnanChinArrayshear-wave splittingseismic anisotropylithospheric couplingasthenospheric flow

We measured shear-wave splitting of teleseismic XKS phases (i.e., SKS, SKKS and PKS) recorded by more than 300 temporary ChinArray stations in Yunnan of SE Tibet. The first-order pattern of XKS splitting measurements shows that the fast polarization directions (ϕ) change (at ∼26–27◦N) from dominant N–S in the north to E–W in the south. While splitting observations around the eastern Himalayan syntax well reflect anisotropy in the lithosphere under left-lateral shear deformation, the dominant E–W ϕ to the south of ∼26◦N is consistent with the maximum extension in the crust and suggest vertically coherent pure-shear deformation throughout the lithosphere in Yunnan. However, the thin lithosphere (<80 km) could account for only part (<0.7 s) of the observed splitting delay times (δt, 0.9–1.5 s). Anisotropy in the asthenosphere is necessary to explain the NW–SE and nearly E–W ϕ in these regions. The NE–SW ϕ can be explained by the counter flow caused by the subduction and subsequent retreat of the Burma slab. The E–W ϕ is consistent with anisotropy due to the absolute plate motion in SE Tibet and the eastward asthenospheric flow from Tibet to eastern China accompanying the tectonic evolution of the plateau. Our results provide new information on different deformation fields in different layers under SE Tibet, which improves our understanding on the complex geodynamics related to the tectonic uplift and southeastward expansion of Tibetan material under the plateau.

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

The Tibetan Plateau (Fig. 1a) due to the continental collision of the Eurasian and Indian plates since ∼50 Ma is the most dramatic plateau in the Earth (e.g., Tapponnier et al., 2001). Different mod-els have been proposed to explain the evolution the plateau, such as the lateral extrusion of the lithospheric materials along major strike-slip faults (e.g., Tapponnier et al., 1982, 2001), the thickening of the Asian crust (England and Houseman, 1989), and the duc-tile flow in the mid-lower crust (e.g., Royden et al., 1997, 2008). The structures and dynamics beneath SE Tibet is important for understanding the tectonic evolution of the Tibetan Plateau. It is characterized as extensive strike-slip faults and the accompanying shear zones along major tectonic boundaries at surface (Fig. 1b). The Ailao Shan–Red River (ASRR) fault zone is the original south-western boundary of South China Block (Fig. 1a) (e.g., Ren, 1999).

* Corresponding author at: Institute of Geophysics and Geodynamics, Nanjing University, Nanjing 210046, China.

E-mail address: ihuangz@nju.edu.cn (Z. Huang).

http://dx.doi.org/10.1016/j.epsl.2015.10.0270012-821X/© 2015 Elsevier B.V. All rights reserved.

The southwestern part of South China (or Yangtze Craton) (i.e., to the west of Xiaojiang Fault at ∼103◦E; also SE Chuan-Dian Block) has been evolved into the active tectonics of SE Tibet, which is indicated by high topography, many active faults (Fig. 1b) and ex-tensive low velocity anomalies in the upper mantle (Huang et al., 2015a).

Seismic anisotropy that results from deformation of the mate-rials in the Earth is essentially important for understanding the deformation styles at different depths (e.g., Karato et al., 2008;Mainprice, 2007; Savage, 1999; Silver and Chan, 1991; Silver, 1996). Many previous studies with teleseismic shear-wave (XKS; i.e., SKS, SKKS and PKS) splitting analysis revealed the first-order pattern of anisotropy in the upper mantle in and around east Ti-bet that the fast polarization rotates clockwise around the eastern Himalayan syntax and changes abruptly from nearly N–S to E–W at ∼26◦N in Yunnan (e.g., Flesch et al., 2005; Huang et al., 2011, 2007; Lev et al., 2006; Sol et al., 2007; Wang et al., 2008, 2013; Zhao et al., 2013b). There are ongoing debates on whether the crust and upper mantle are decoupled or not based on the com-parison between surface deformation field revealed by GPS and

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Fig. 1. (a) Tectonics in and around the Tibetan Plateau. Red arrows denote the crustal motion revealed by GPS observations (Gan et al., 2007). The black arrow denotes the motion of the Indian Plate relative to the stable Eurasian Plate (Argus et al., 2011). Gray curves show major tectonic boundaries in and around the plateau (Ren, 1999). (b) Distribution of the 343 portable broadband stations (inverted blue trian-gles) deployed by the ChinArray project. The Chuan-Dian Block (CDB) is surrounded by major faults, i.e., Xianshuihe fault (XSHF) to the north, Jinshajiang fault (JSJF) to the west, Ailao Shan–Red River fault (ASRR) to the south, and Xiaojiang fault (XJF) to the east. The red triangle shows Tengchong (TC) volcano. The red curves denote active faults. EHS: eastern Himalayan syntax; LMS: Longmenshan fault; YJO: You-jiang orogen. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

geological observations and upper mantle deformation field re-vealed by XKS splitting measurements (e.g., Flesch et al., 2005;Sol et al., 2007; Wang et al., 2008; Chang et al., 2015). Moreover, recent seismic images and numerical simulation indicate extensive asthenospheric flow extruded from the plateau to eastern China (e.g., Huang et al., 2015a, 2015b; Li et al., 2008; Liu et al., 2004;Zhang et al., 2014), which could induce significant anisotropy in the asthenosphere and makes the XKS splitting observations more complex than expected.

Previous XKS splitting parameters were measured at perma-nent and temporary stations with lateral spacing of generally 50–100 km. In this study, we measured shear-wave splitting of teleseismic XKS phases recorded by more than 300 temporary sta-tions (with lateral spacing of ∼30 km) deployed by ChinArray project in Yunnan of SE Tibet, which provides more information on the lateral variations of XKS splitting observations and thus the

Fig. 2. Distribution of the 67 events used in this study. The red circles, blue squares, and green stars denote the events whose SKS, SKKS, and PKS phases are used, re-spectively. The magenta curves denote the plate boundaries (Bird, 2003). The four great circles denote the epicentral distances of 85◦ , 90◦ , 120◦ , and 150◦ from the study region. The red rose diagram in the central inset shows the statistic of the back-azimuths of the events relative to the stations (inverted blue triangles). (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

crust and upper mantle anisotropy. We then compared the XKS splitting measurements with crustal deformation field (from GPS and geological observations and focal mechanism solutions) and anisotropy (by Pms splitting), and P wave velocity anomalies in the asthenosphere. The results indicate that the lithosphere under Yunnan of SE Tibet is probably suffering vertically coherent defor-mation. The anisotropy in the lithosphere explains a large portion of the XKS splitting observations, but significant anisotropy in the asthenosphere is necessary to explain measurements especially in south Yunnan with thin lithosphere.

2. Data and method

2.1. Data

The waveform data used in this study is recorded by 343 portable stations (Fig. 1b) of the ChinArray project deployed in SE Tibet (mostly in Yunnan province) during one year from Au-gust 2011 to August 2012. Most of the stations were equipped with a Guralp CMG-3EPC three-component broadband seismome-ter and a Reftek-130 digitizer. The sampling rates are 100 samples per second. We selected 67 events (Fig. 2) with magnitude >5.8 and epicentral distances of 88◦–140◦ and used their core phases (XKS) for shear-wave splitting analysis. In general, the SKS and SKKS phases are clear for events with epicentral distances <120◦while the PKS phases become dominant for events with epicen-tral distances >130◦ . Most of the events occurred in the Tonga and New Zealand subduction zones in Southwest Pacific and in the subduction zones in North America. Some events occurred in the mid-ocean ridge in the Atlantic Ocean and Indian Ocean. The events are generally concentrated in very narrow back-azimuths (Fig. 2), which is actually not ideal for shear-wave splitting anal-

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Fig. 3. An example of shear-wave splitting measurement on an SKS phase. (a) The dashed blue and solid red curves denote the initial radial (Q) and transverse (T) components of the waveform, respectively, which were firstly filtered with a 3rd order Butterworth bandpass filter in the range of 0.02–0.125 Hz. The gray region shows the waveform window selected for shear-wave splitting analysis. The information about the event, filter range, signal-to-noise ratio (SNR) of the Q and T components, and dominant frequency of the Q and T components is also shown. (b) The map of energy on the T component in the ϕ − δt space in the grid search. The red cross shows the optimal splitting parameters accounting for minimum T component while the gray region denotes the 95% confidence region. The black triangles denote the back-azimuth of the event and its orthogonal direction. The information about the station, selected phase, and optimal (ϕ, δt) pair is shown at the top. (c) Corrected Q (dashed blue curve) and T (red solid curve) components after correcting the splitting effect. (d) Initial (dashed blue curve) and corrected (red solid curve) particle motions. We also show the ratios between the transverse (T) and radial (Q) component before (Ini.) and after (Fin.) the shear-wave splitting analysis, as well as their ratio (Fin./Ini.). (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

ysis especially when it is necessary to characterize the azimuthal variations of shear-wave splitting observations.

2.2. Method

Shear-wave splitting describes the phenomenon that a shear wave splits into two shear waves with orthogonal polarizations and different velocities when traveling through an anisotropic medium. Two splitting parameters, i.e., the polarization of the fast wave (ϕ) and the delay time (δt) between the fast and slow waves, are used to describe the shear-wave splitting quantitatively. We used transverse-component minimization method (Silver and Chan, 1991; Walsh et al., 2013) conducted in the SplitLab tool-box (Wüstefeld et al., 2008) to measure the splitting parameters from the seismograms directly. It utilizes a grid-search approach to identify the best-fitting splitting parameters that minimize the energy in the transverse component by rotating (−90◦ to 90◦with a step of 1◦) and time-shifting (0 s to 4.0 s with a step of 0.02 s) the Q–T components in a ray-coordinate based L–Q–T coordinate system (e.g., Vecsey et al., 2008). The uncertainties are estimated by 95% confidence region of F-test with the up-dated calculation of the degrees of freedom (Walsh et al., 2013). Fig. 3 shows an example of shear-wave splitting measurement on an SKS phase. It is very convenient to evaluate the results preliminary by estimating the original seismograms (Fig. 3a), cor-rected T component (Fig. 3c), initial and corrected particle motions (Fig. 3d) and 95% confidence region (Fig. 3b) simultaneously. The transverse-component minimization method is more stable than the other two popular techniques that either maximizes the corre-

lation of the fast and slow components or minimizes the smaller eigenvalue of the covariance matrix (e.g., Vecsey et al., 2008;Wüstefeld and Bokelmann, 2007) when noise is present. We did not apply the stacking technique described in Wolfe and Silver(1998) or Restivo and Helffrich (1999) in the present study. On the one hand, we use many strict criteria on both the initial and corrected waveforms (see section below) to make sure that the results are reliable. Especially, the signal-to-noise ratios (SNR) of the initial seismograms are important to make reliable shear-wave splitting analysis (Restivo and Helffrich, 1999). On the other hand, the stacking technique was firstly proposed to determine splitting parameters at stations in oceanic regions where anisotropy could be well modeled with one simple layer (Wolfe and Silver, 1998). As showed later, the anisotropy in SE Tibet is complex, proba-bly located in both lithosphere and asthenosphere due to different mechanisms; the stacking technique may therefore hide some im-portant features.

3. Results and analysis

3.1. Results

We obtained a total of 5921 preliminary shear-wave splitting measurements after visual selection. Following previous studies (e.g., Liu and Gao, 2013; Vecsey et al., 2008), we first rank these results on the basis of the SNR values on the Q and T compo-nents (Figs. 4a and 4b), the angular difference between ϕ and back-azimuth (Fig. 4c), the T/Q ratios of the initial and corrected particle motions (Figs. 4d and 4e) as well as their ratio that de-

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Fig. 4. Histograms of the signal-to-noise ratios of the Q (a) and T (b) components, the angular between ϕ and back-azimuth of the event (c), the T-to-Q ratio before (d) and after (e) the shear-wave splitting analysis as well as their ratio (f). The vertical bars with horizontal arrows denote the criteria to define the good and fair non-Null and Null measurements in this study.

scribes whether the splitting effect is corrected efficiently (Fig. 4f). Accordingly, we obtained 90 good and 435 fair non-null measure-ments (Table S1), 349 good and 827 fair null measurements, and >70% poor measurements that are not discussed in the study.

Fig. 5a show the good and fair non-null shear-wave splitting measurements in the study region. The overall uncertainties of ϕand δt are generally <20◦ and <0.6 s (Table S1), respectively. The regional-average patterns are notable (Fig. 5a) and suggest reliable measurements. As a whole, ϕ aligns mostly N–S to NW–SE in the north and changes to dominant E–W in the south. However, the results of the western and eastern regions show visible variations. In the western region (to the west of ∼103◦E), the XKS splitting measurements show dominant transition at ∼26◦N from N–S ϕin the north to E–W ϕ in the south. NW Yunnan (or NW Chuan-Dian Block) is mainly part of the Tibetan Plateau; ϕ aligns mainly NNW–SSE in spite of some nearly E–W observations (e.g., station 51054 in Fig. 5a). SW Yunnan belongs to the Indochina block and ϕ generally aligns E–W to NE–SW. In west Yunnan (between 24◦N and 26◦N) and the SE Chuan-Dian Block, the dominant ϕ is nearly E–W while some NW–SE ϕ observations are visible to the west of the Tengchong (TC) volcano. Similarly, in the eastern region (to the east of ∼103◦E), the observations are divided to the NE area with dominant NW–SE ϕ , east Yunnan with mainly E–W ϕ , and SE Yun-nan with E–W to NEE–SWW ϕ . An important result is that, as the first-order feature, the abrupt change of ϕ (e.g., from N–S in the north to E–W in the south) occurs at different latitudes, i.e., it is visible at 26◦N in the western region but at 27◦N in the eastern region (Fig. 5a).

The directions of the nulls are generally consistent with the back-azimuths of the events (Figs. 2 and 5b) rather than the ϕdirections. For example in the SE area that belongs to the bound-ary orogen between the Yangtze and Cathaysia cratons, the null directions (NNE–SSW or SEE–NWW) are at ∼45◦ angle from the ϕ directions (NEE–SWW), which is actually incompatible. The re-sults may reflect extreme lateral heterogeneity that is not coherent over the length scales associated with the seismic wavelengths un-der study (e.g., Eakin et al., 2015). On the other hand, in several stations especially close to the Tengchong volcano, the null mea-surements cover a large back-azimuthal range, which may indicate

isotropy beneath the stations because the splitting is smaller than the lower detection limit of the applied method.

To check whether anisotropy in the deeper mantle (e.g., tran-sition zone and D′′ layer) influences the general pattern of the splitting measurements (Fig. 5a), we plot the measurements of 9 single events at all the stations (with at least 20 good and fair non-null measurements) (Fig. 6). The first-order pattern of the splitting measurements, i.e., change from dominant N–S ϕ in the north to dominant E–W ϕ in the south, is evident for measure-ments of both events from the northern (Fig. 6a–d) and southern (Fig. 6f) quadrants. Because the raypaths at different stations from the same event are almost the same in the transition zone and D′′layer taking into account the finite-frequency effect, the observed lateral variations of the splitting measurements (Fig. 6) are better interpreted as the anisotropy in the crust and upper mantle. The results are consistent with previous conclusion that the anisotropy is mainly located at <160 km depths based on Fresnel zone anal-ysis (e.g., Lev et al., 2006; Sol et al., 2007).

The delay times (δt) reflect either the strength of anisotropy or the thickness of anisotropic layer under the stations. In the study region δt mainly ranges between 0.6 and 2.1 s (Fig. 5a) with peaks at 0.9–1.5 s. The observations indicate the thickness of the anisotropic layer of dominantly 100–170 km for typical upper mantle anisotropy (e.g., Silver and Chan, 1991; Savage, 1999). The δt values seem larger (∼1.5 s) at stations in the stable Yangtze (NE area) and Cathaysia (SE area) cratons. In contrast, they are smaller (∼1.0 s) in regions with complex structures (e.g., west Yunnan with many active faults).

3.2. Compared with previous studies

There have been many XKS splitting measurements in SE Tibet using the waveforms recorded by either permanent (e.g., Huang et al., 2011; Wang et al., 2008, 2013; Zhao et al., 2013b) or portable stations (e.g., Flesch et al., 2005; Huang et al., 2007;Lev et al., 2006; Sol et al., 2007; Chang et al., 2015). The most dominant feature, i.e., the change of ϕ from roughly N–S in the north to E–W in the south, is significant in all the previous and the present studies.

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Fig. 5. (a) The 90 good (red bars) and 435 fair (blue bars) non-null XKS splitting measurements obtained in the studies. The orientations of the short bars denote the polarizations of fast shear waves (ϕ) while the lengths of the short bars show the delay time (δt) between the fast and slow waves. The reference for δt is shown in the bottom-left inset. The colored background shows the distribution of δt with the scale shown in the bottom-right inset where the histogram of δt is shown simultaneously. The orange curves denote the active faults in the study region. (b) Distribution of the 349 good and 827 fair null measurements in our data set. Nulls are plotted as crosses at each station with the bars orientated in the back-azimuth and its orthogonal direction. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

Nevertheless, we reveal some more details in the present study thanks to the denser stations (with lateral spacing of ∼30 km) compared with previous studies (∼50–100 km). For instance, we found that the abrupt change of ϕ occurs at different latitudes in the western and eastern regions. ϕ changes the direction at ∼26◦N in the western region while at ∼27◦N in the easternregion (Fig. 5a). Otherwise, we obtained NW–SE and even N–S ϕ to the west of the Tengchong volcano as well as some NE–SW ϕ in SW Yunnan. Therefore, our study provides more information and constraints on the upper mantle structures and dynamics in SE Tibet.

4. Discussion

Anisotropy is a characteristic feature of the Earth’s materials in most depths (see Mainprice, 2007 for review). In the upper crust with brittle deformation, the anisotropy is mainly caused by the alignment of cracks (i.e., shaped-preferred orientations, SPO) (e.g., Crampin, 1984). The fast orientation generally reflects the long axes of the cracks, which is parallel to the maximal stress in tectonically-stable areas but changes to be consistent with the active faults and boundaries in tectonically-active areas. In the lower crust and upper mantle that are in plastic deformation, the anisotropy is caused by the lattice-preferred orientations (LPO) of minerals, such as mica in the lower crust and olivine in the upper mantle (e.g., Mainprice, 2007; Silver, 1996). The fast orientation is generally parallel to the maximum shear or the extensional axis, which would change to be consistent with active faults and tec-tonic boundaries (e.g., Silver, 1996; Savage, 1999). For simple man-tle flow in the asthenosphere, the fast orientation generally reflects the mantle flow direction (e.g., Karato et al., 2008). Additionally, the metamorphic rocks also induce significant anisotropy with the fast orientation parallel to the lineation and foliation (e.g., Ji et al., 2015 ).

4.1. Anisotropy in lithosphere

The contribution of crustal anisotropy to the XKS splitting ob-servations could be constrained with Pms splitting analysis (e.g., Sun et al., 2012, 2013). The ϕ of the Pms splitting show visi-ble correlation with the surface structures (e.g., faults). Therefore the anisotropy in the crust mainly reflects the SPO of the cracks (Crampin, 1984) and metamorphic rocks near the active faults (e.g., Ji et al., 2015). The dominant δt is ∼0.3 s (Fig. 7a, inset) although two measurements in NW Yunnan with thick crust (∼60 km) has larger δt values (∼0.6–0.8 s) (Fig. 7a). Yao et al. (2010) predicted the δt of crustal anisotropy from their azimuthally anisotropic model inverted from surface-wave and showed that the values are mostly <0.3 s, which is consistent with the Pms splitting. There-fore, as a whole, the anisotropy in the crust accounts for only ∼20% of the total XKS splitting (Fig. 7a) and the observations mainly reflect the anisotropy in the mantle lithosphere and the asthenosphere below.

Previous studies compared the crustal deformation field in-ferred from GPS and geological data and mantle deformation field inferred from XKS splitting measurements, which raised debate on whether the crust and upper mantle are decoupled or not beneath the Tibetan Plateau (e.g., Chang et al., 2015; Flesch et al., 2005;Sol et al., 2007; Wang et al., 2008). All these studies confirmed that the crust and upper mantle are mechanically coupled around the eastern Himalayan syntax (to the north of 26◦N). The anisotropy due to dominant left-lateral shear deformation throughout the lithosphere (Fig. 7b, green bars) well explain XKS splitting mea-surements while extent of large-scale lower crustal flow (e.g., Roy-den et al., 1997, 2008) is not necessary. But in south Yunnan (to the south of 26◦N), nearly E–W ϕ of XKS splitting observations is different with that predicted by the left-lateral shear in the crust from GPS and geological data. Therefore the crust and up-per mantle are probably decoupled and the crustal and mantle deformations are different (Flesch et al., 2005; Sol et al., 2007). The model argues that mantle deformation is mainly controlled by boundary conditions; crustal buoyancy forces are not transmitted into the mantle (Flesch et al., 2005). But later study showed that ϕ of XKS splitting measurements is consistent with the maximum extension in the crust inverted from GPS and geological data (e.g., Wang et al., 2008) (Fig. 7b, pink bars) and earthquake focal mecha-nism solutions (e.g., Zhao et al., 2013a) (Fig. 7b, yellow bars). Thus

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Fig. 6. Splitting parameters measured on 9 PKS and SKS phases recorded by all the stations simultaneously. See Fig. 5 for the other labeling.

the crust and mantle could be coupled and suffering similar de-formation. In the crust the strain field arises from the topographic gradient-parallel shortening under gravitational potential (Fig. 1b) while in the upper mantle it is formed when the collapsing litho-sphere encounters resistance from the surrounding medium (e.g., Wang et al., 2008). We favor the later model that the crust and mantle are coupled. Firstly, in the decoupled crust–mantle model, the crust is expected to move southward with respect to the man-tle at rates as high as ∼30 mm/yr (Flesch et al., 2005), which causes extensive shear and thus significant anisotropy at the base of crust. Therefore, the XKS especially Pms splitting observations with nearly N–S ϕ should be visible, which is not supported by the actual observations (Figs. 5 and 7a). Secondly, receiver func-tion analysis showed the Vp/Vs ratios of the crust beneath south Yunnan is in normal or even smaller values (Sun et al., 2012;Wang et al., 2010), which does not support a ductile lower crust there.

In the NE corner, ϕ of XKS splitting measurements changes abruptly at ∼27◦N, from NW–SE in the north to dominantly E–W in the south (Fig. 5). Both directions are different with the surface structures and crustal deformation field (Fig. 7b). The observations with NW–SE ϕ are mostly located in the core of the stable Yangtze Craton, where is characterized as very thick lithosphere (Pasyanos et al., 2014) (Fig. 8a) and higher P wave velocity in the upper mantle (Huang et al., 2015a) (Fig. 8b). The Yangtze Craton is a Pre-cambrian craton covered by widespread Proterozoic and Archean rocks (e.g., Zheng et al., 2006). The core of the Yangtze Craton (i.e., Sichuan Block) has not suffered extensive deformation although the surrounding regions may be involved into the active tecton-ics during long geological history (Pirajno, 2013). Thus anisotropy here is most likely fossil anisotropy from past geological processes when the craton was formed but not due to the present mantle de-

formation (e.g., Wang et al., 2013). The thickness of the lithosphere is larger than 160 km (Fig. 8a), which could produce δt of >1.4 s (Silver and Chan, 1991; Silver, 1996) and completely account for the XKS splitting observations (Fig. 5a). Similar fossil anisotropy may also exist in SE area that is a Hercynian–Indosinian orogen between the Yangtze and Cathaysia cratons (Ren, 1999). The fast orientations of the fossil anisotropy are consistent with the strike of the orogen (i.e., NE–SW). Anisotropy due to present deformation in the lithosphere arising from the growth of the plateau (Fig. 7b) is similar to the fossil anisotropy. They impose on the original anisotropy so that the XKS splitting measurements are more stable and have relatively larger δt (Fig. 5a)

However, in central and south Yunnan, the thickness of litho-sphere is smaller than 80 km (Fig. 8a), which produces anisotropy of only <0.7 s (Silver and Chan, 1991; Silver, 1996). It is not capa-ble of explaining the XKS splitting measurements mainly ranging between 0.9 s and 1.5 s (Fig. 5a). Up to as much as 50% XKS splitting in some areas cannot be explained by anisotropy in the lithosphere. Thus, anisotropy in the asthenosphere must be con-sidered (e.g., Wang et al., 2013).

4.2. Anisotropy in asthenosphere

In SE Tibet, anisotropy in the asthenosphere mainly arise from three geodynamic processes. The first one is the absolute plate mo-tion (APM) of the rigid lithosphere (Fig. 8) (Argus et al., 2011) that induces extensive shear deformation in the top of the astheno-sphere (e.g., Silver, 1996; Savage, 1999). The ϕ of anisotropy in this case is parallel to the APM direction of the Eurasian Plate, i.e., ∼110◦ clockwise from north in Yunnan (Fig. 8). The second tec-tonic process is the asthenosphere flow extruded from Tibet to eastern China (e.g., Huang et al., 2015a, 2015b; Li et al., 2008;

360 Z. Huang et al. / Earth and Planetary Science Letters 432 (2015) 354–362

Fig. 7. (a) Comparison between present XKS splitting measurements (red and blue bars) and Pms splitting measurements (Sun et al., 2012, 2013). The gray colors with contours in the background denote the thickness of the crust obtained by receiver function analysis (Sun et al., 2012; Wang et al., 2010) with scale shown in the bottom-left inset. The yellow and cyan histograms in the bottom-right inset show the statistics of δt for the Pms and XKS splitting measurements, respectively. See Fig. 5 for the other labeling. Note that the length scale of δt for Pms splitting measurements are twice as those for XKS splitting measurements. (b) Comparison between XKS splitting measurements (red and blue bars), left-lateral shear deforma-tion (green bars) (Flesch et al., 2005) and maximum extension (pink bars) (Wang et al., 2008) inverted from GPS and geological observations, and σ3 orientations (T-axis) (yellow bars) (Zhao et al., 2013a) inverted from earthquake focal mechanism solutions. For the other labeling, see Fig. 5. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

Wei et al., 2012). The ϕ of anisotropy is sub-parallel to the as-thenosphere flow (e.g., Karato et al., 2008) which is nearly E–W ex-tending from Yunnan to South China eastward. The third process is related to the subduction of the Burma Plate and its westward re-treat at present (Fig. 8) (e.g., Huang and Zhao, 2006; Li et al., 2008;Ni et al., 1989; Wang et al., 2013; Wei et al., 2012). The local

Fig. 8. (a) Comparison between XKS splitting measurements (red and blue bars) and thickness of the lithosphere (Pasyanos et al., 2014). The color scale for the thickness of the lithosphere is shown at the bottom. The black arrows show the retreat of the Burma slab (Ni et al., 1989) and the absolute plate motion (APM) in SE Tibet (Argus et al., 2011). The rose diagram in the bottom-left inset show the statistics of ϕ to the south of 26◦N. (b) Comparison between XKS splitting measurements (red and blue bars) and P-wave velocity anomalies at 150 km depth (Huang et al., 2015a). The scale for P-wave velocity anomalies is shown in the top-right inset. For the other labeling, see Fig. 5. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

mantle convection due to the subduction and subsequent retreat induce anisotropy with NE–SW ϕ in the asthenosphere beneath Yunnan and South China (Wang et al., 2013). Overall, the first and second mechanisms generate similar anisotropy with nearly E–W ϕ while the third process produce the anisotropy with NE–SW ϕ . We actually observed two corresponding groups of dominant E–W and NE–SW ϕ in regions with lithosphere <80 km (i.e., to the south of 26◦N) (Fig. 8a, inset rose diagram) where anisotropy in the asthenosphere may play important roles.

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Two XKS splitting measurements in SW Yunnan (Figs. 5a, 7and 8; station 53 094 and 53 100) have larger δt of the order of ∼2.0 s. It is consistent with the strikes of active faults and the crustal deformation field of left-lateral shear (Flesch et al., 2005). The crustal anisotropy also shows the same NE–SW ϕ revealed by Pms splitting (Fig. 7a) (Sun et al., 2012, 2013) and P-wave anisotropy tomography (Huang et al., 2014; Wei et al., 2013). How-ever, the 80 km thick lithosphere can account for at most 0.7 s XKS splitting; the remaining δt of >1.0 s should be produced in the asthenosphere. Seismic tomography studies clearly revealed the eastward dipping Burma slab in the upper mantle beneath SE Tibet (Huang and Zhao, 2006; Huang et al., 2015a, 2015b; Li et al., 2008;Ni et al., 1989; Wei et al., 2012). A zone of significant lower P wave velocity anomaly is visible under south Yunnan extending from 100 km to 300 km depths (Huang and Zhao, 2006; Huang et al., 2015a, 2015b; Li et al., 2008; Wei et al., 2012), suggesting smaller-scale counter flow in the upper mantle with warmer slower mate-rial driven by the retreat of the Burma slab (e.g., Sol et al., 2007;Wang et al., 2013).

In east Yunnan, the abrupt change of ϕ at ∼27◦N (from NW–SE in the north to nearly E–W in the south) (Fig. 8) argues for significant asthenospheric flow around the core of the Yangtze Cra-ton (i.e., Sichuan Block). As discussed above, the measurements with NW–SE ϕ reflect the fossil anisotropy in the craton. The anisotropy with E–W ϕ is inconsistent with anisotropy in the lithosphere (probably with NE–SW ϕ) (e.g., Wang et al., 2013) revealed by either the crustal deformation field of maximum ex-tension (Fig. 7b) (Wang et al., 2008; Zhao et al., 2013a) or P wave anisotropy tomography (Wei et al., 2013). What is more impor-tant, the abrupt change occurs at a transition zone from thick (∼120 km) to thin (∼80 km) lithosphere (Fig. 8a) (Pasyanos et al., 2014) and from high (positive) to low (negative) P wave ve-locities at 150 km depths (Fig. 8b) (Huang et al., 2015a). In this region, the root of the Yangtze Craton has been destructed by the asthenospheric flow extruded from the Tibetan Plateau (Huang et al., 2015a). Thus the XKS splitting observations with nearly E–W ϕare best explained by the anisotropy due to the eastward extrusion of the asthenosphere (e.g., Huang et al., 2015a; Liu et al., 2004;Zhang et al., 2014). Further P wave anisotropy tomography (Huang et al., 2015b) and XKS splitting analysis (e.g., Wang et al., 2013) in South China indicate that the asthenospheric flow extends to South China Sea, which may be closely related to the evolution of the South China Sea and the Hainan plume (e.g., Huang et al., 2015b;Li et al., 2008).

5. Summary

We measured shear-wave splitting of teleseismic XKS phases (i.e., SKS, SKKS and PKS) recorded by more than 300 temporary stations of ChinArray deployed in Yunnan of SE Tibet. The dense stations with lateral spacing of ∼30 km revealed more details of the anisotropic structures, which provide new information for un-derstanding the complex deformation and dynamics in the upper mantle under SE Tibet.

Comparison between shear-wave splitting measurements and crustal deformation field inferred from GPS and geological observa-tions and earthquake focal mechanism solutions indicates different mechanisms in and off the Tibetan Plateau. Around the eastern Hi-malayan syntax, the splitting measurements (with NW–SE ϕ) can be explained by the lithospheric anisotropy by vertically coherent left-lateral shear deformation. In contrast in Yunnan off the plateau (to the south of ∼26◦N), the splitting measurements (with E–W ϕ) are consistent with the maximum extension under N–S short-ening in the crust, indicating the crust and mantle may be coupled and the decoupling model is not necessary. Nevertheless, the thin lithosphere (<80 km) could only explain part (<0.7 s) of the total

XKS splitting δt (0.9–1.5 s) there while the remaining anisotropy is produced in the asthenosphere. In the core of the stable Yangtze Craton (Sichuan Block) in the NE area, the observations (with NW–SE ϕ) reflect fossil anisotropy in the Precambrian craton of very thick lithosphere (>160 km).

Anisotropy in the asthenosphere is very important in the study region because the lithosphere is generally too thin (<80 km) to generate the XKS splitting with dominant δt of 0.9–1.5 s. Two im-portant groups of ϕ are visible in the measurements. One group includes mainly NE–SW ϕ that reflect smaller-scale counter flow in the upper mantle driven by the subduction and subsequent re-treat of the Burma slab. Another group consists of nearly E–W to NWW–SEE ϕ that result from anisotropy duo to the APM direction of the Eurasian Plate in SE Tibet and the eastward asthenospheric flow extruded from Tibet to eastern China.

In spite of the new results and their implications for the com-plex structures in SE Tibet, more work focusing on revealing depth-dependent anisotropy is very important for better understanding the crust and upper-mantle dynamics. The data from temporary deployments of only one year is not enough for interpreting multi-layer anisotropy. So analyzing the data in longer period or even permanent stations is very important. Another potential improve-ments include inverting high-resolution 3-D azimuthal anisotropy with the data of the dense ChinArray stations by P wave (e.g., Huang et al., 2015a, 2015b; Wei et al., 2013) and surface wave anisotropic tomography (Yao et al., 2010).

Acknowledgements

This work was supported by China National Special Fund for Earthquake Scientific Research in Public Interest (201008001, 201308011), Key Laboratory of Seismic Observation and Geophys-ical Imaging (SOGI 2013 FUDA01), and National Natural Science Foundation of China (41204040). Prof. An Yin (the Editor) and two anonymous reviewers provided constructive comments and sug-gestions that greatly improved the manuscript. We are grateful to the High Performance Computing Center of Nanjing University for doing the numerical calculations in this paper on its IBM Blade cluster system. Most figures were made using GMT (Wessel et al., 2013).

Appendix A. Supplementary material

Supplementary material related to this article can be found on-line at http://dx.doi.org/10.1016/j.epsl.2015.10.027.

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